Butanol is a high quality fuel and fuel additive. Butanol can be mixed, stored and transported together with gasoline. It has more energy per gallon than ethanol, which translates into better fuel economy for consumers using butanol blends, and has lower vapor pressure than ethanol, which translates into less ground level pollution. Butanol's low vapor pressure makes it an attractive low volatility, oxygenated, blend component for refiners to use in complying with stringent vapor pressure specifications. Butanol can provide the oxygenate benefits of ethanol but without undue evaporative emissions, which are a significant source of air pollution, and at a potentially lower cost. Butanol is also more hydrophobic than ethanol, i.e., it has a higher tendency to repel water, and is more suitable for blending with gasoline. As such, butanol should be a highly desired component of Reformulated Gasoline Blendstock for Oxygenate Blending (RBOB) and California (CARBOB) fuel blendstock. Butanol is also expected to have a reduced life cycle emission of CO2. Butanol blends should have no detrimental effects on modern fuel system elastomers, and corrosion and electrical conductivity are expected to be similar to gasoline.
Butanol is also widely used as an industrial chemical. It is used in the production of paints, plasticizers, and pesticides, as an ingredient in contact lens cleansers, cement, and textiles, and also as a flavoring in candy and ice cream. The global market for n-butanol was approximately 1 billion gallons in 2006; the U.S. market was approximately 300 million gallons, and is expected to grow approximately 2% per year.
Butanol is currently made from petroleum. Production costs are high and margins are low, and price trends generally track the price of oil and are heavily influenced by global economic growth. There is a need for improved methods for production of butanol. In particular, methods for environmentally compatible, cost efficient, and energy efficient production of butanol would be desirable.
Industrial scale fermentations were historically performed for solvent and acid production prior to the rise of the petrochemical industry. Concerns about pollution, climate change, and resulting environmental degradation have renewed interest, particularly where low cost or waste biomatter are available as feedstock. One problem that economically constrains more widespread adoption is the high energy expenditure required to recover fermentation products from the low concentrations typically seen in fermentation broths. Efforts to increase product concentrations in fermentation broths have met with limited success owing to the toxicity of these compounds to the cultured microorganisms. Another issue which constrains the economic feasibility of fermentation based bioproducts is the productivity of the fermentation process. Increases in productivity lead to an improved use of installed capital.
Methods for producing microbial strains with improved butanol tolerance and fermentation titer have involved selection in butanol-containing liquid culture medium or on butanol-containing agar plates. Liquid medium or plates containing high butanol concentrations release butanol vapors through evaporation, which causes unfavorable environmental conditions for workers. Evaporation also makes it difficult to regulate the concentration of butanol in plate or liquid culture. There is a need for an improved method for selecting microbial strains with increased butanol tolerance or titer using a screening method that does not include addition of butanol to the growth medium.